Physostigmine, also called eserine, and particular derivatives of physostigmine are anti-cholinesterase inhibitors which are well known. Such well known compounds are also useful in the treatment of glaucoma, Myasthenia Gravis, Alzheimer's disease and as antidotes against poisoning with organophosphates.
It has been discovered that the natural isomer of physostigmine has blocking properties as well as agonist properties at the neuromuscular AChR. By contrast (+)-physostigmine shows only negligible inhibition of cholinesterase (ChE). See Brossi et al., FEBS Lett., Vol. 201, pages 190-192 (1986).
Even though (+)-physostigmine has only negligible ChE inhibitory activity, it is as effective as a protective pretreatment drug against multiple lethal doses of sarin, see Albuquerque et al, Fundam. Appl. Caltoxicol., Vol. 5, pages 182-203 (1985). The observed beneficial protection appears to be due to direct interactions of the carbamates with the postsynaptic nicotinic AChR. The protective effectiveness of the carbamates against organo-phosphates appears to be related to the direct ability of the carbamates to decrease the hyperactivation caused by accumulation of the neurotransmitter.
The above information, available due to the research in this field, is important in the evaluation of potential new pharmacological agents for treating cholinergic disorders, for example, Myasthenia Gravis and Alzheimer's disease. Potential agents can be evaluated for potency in vitro by testing the agents against electric eel acetylcholinesterase (AChE) and human plasma butyrylcholinesterase (BChE).
Of the two enzymes known to hydrolyze acetyl-choline (ACh) in vivo, AChE, which is found in red blood cells, in the brain and in nerve tissues, seems to be more specific than BChE which is found in serum, pancreas and in the liver. It, however, has not previously been shown in the art that compounds which selectively inhibit one of the two enzymes more than the other would offer a medical advantage. The natural alkaloid (-)-physostigmine, its potential metabolite (-)-(N1)-norphysostigmine, and the natural alkaloid physovenine which are used as biological standards in this art area, inhibit AChE and BChE in vitro similarly at similar concentrations.
Accordingly, there is need in the art for highly selective agents active against one of AChE and BChE while not being potent against the other so as to lead to better treatment of a particular cholinergic disorder and minimize negative side effects. Such compounds would be of great medical importance in the treatment of cholinergic disorders.